The invention described below generally relates to an excitation system for rotating synchronous machines, and more particularly, to a current-source excitation system for rotating synchronous machines that employ a superconducting field winding.
Synchronous electric machines, including motors and generators, are employed in many applications requiring conversion of electrical power to mechanical power or mechanical power to electrical power. Synchronous motors, for example, may provide mechanical power for operation of factories, movement of materials, and a variety of other applications. Conversely, synchronous generators may provide electrical power in many similar applications. In order to effectively operate synchronous machines, control and excitation systems arc typically employed.
A particular aspect of synchronous machine operation is to control field-winding current of the machine. Synchronous machines often include a rotating field winding which is supplied by a DC source of power from an excitation system. With synchronous motor operation, the field winding current is typically controlled to adjust reactive power consumed by the motor. With synchronous generator operation, field-winding current is typically controlled to regulate generator output voltage and/or power factor.
Another aspect of synchronous machine operation pertains to the manner in which DC power is supplied to the field winding by the excitation system. Many conventional excitation systems employ brushes and slip rings to supply DC power to the field windings. Conventional excitation systems may eliminate the need for brushes and slip rings by employing a rotating excitation system. If conventional excitation systems are applied to rotating synchronous machines employing superconducting field windings, however, serious machine performance problems may arise due to the unique characteristics of superconducting windings.
In conventional synchronous machines, the field winding often has a relatively large winding resistance and a relatively short time constant. Therefore, conventional voltage-source excitation systems may be employed to control the field winding current. In contrast, superconducting machines typically have a very low resistance in the field winding and a relatively long time constant. Consequently, if control voltages are applied to superconducting field windings with conventional voltage-source excitation systems, significant problems may arise. For example, if voltages are applied to superconducting field windings on the order of the steady-state field winding voltage, it would take a very long time to achieve a steady-state operating current as a result of the long field winding time constant. In an example superconducting field winding with a field winding inductance of 7 Henry and a field winding resistance of 0.005 xcexa9, the field winding time constant would be 1400 seconds. Therefore, excitation voltages several orders of magnitude greater than the steady-state operating voltages are required to rapidly change the field current. Conventional field windings, in contrast, have much shorter time constants, on the order of 2 to 4 seconds, and may only require excitation voltages of three to four times the steady-state operating voltage. Consequently, voltage-source excitation systems are inadequate to control rotating superconducting field windings.
Another challenge presented by the low field-winding resistance of superconducting machines is that small changes in field-winding voltage may produce large changes in field winding current. For example, with the above inductance and resistance, and if 100 Amperes were flowing, a steady state voltage of 0.5 Volts would be maintained across the field winding. If just a 0.2 Volt change occurred across the field winding, a large change in field current (40 Amperes) would result. Voltage changes may occur because of environmental changes such as temperature and/or from normal machine wear itself. Still yet another challenge in superconducting machines is that simply reducing the field winding voltage will not reduce the field current in acceptably reasonable amounts of time.
In view of the above issues, it would be desirable for a brushless excitation system which can accurately and rapidly control field winding current in a rotating synchronous machine.
The present invention provides a current-source excitation system to produce and control the field winding current in rotating synchronous machines. In the case of a rotating synchronous machine employing superconducting field windings, the excitation system provides DC excitation power to the superconducting field winding and eliminates the need for slip rings and brushes. A brushless implementation of the excitation system provides for lower cost and maintenance of the synchronous machine. Additionally, the present invention provides a system for inverting the field winding voltage which allows for rapid decreases in the field winding current.
More particularly, the present invention employs a current-source control system to provide closed-loop control of the field winding current. Precise and rapid control of the field winding current is achieved by applying DC voltage to the field winding, monitoring the resultant field current and controlling the current using feedback in a closed-loop system. Negative and positive DC voltages are provided by a rectification system that is controlled by the current-source control system. The rectification system also allows for brushless implementation of the excitation system.
In accordance with one aspect of the present invention, a system for providing DC current to a rotating winding is provided. A control system receives current feedback from the winding. The control system determines an error signal based on the current feedback and a reference signal. The control system determines a control signal corresponding to the error signal, and provides at least one of a positive and negative excitation voltage to the winding based on the control signal.
Another aspect of the present invention includes a system for providing DC excitation voltages to a rotating superconducting winding; including: means for receiving current feedback from the superconducting winding; means for determining an error signal based on the current feedback and a reference signal; means for determining a control signal corresponding to the error signal; and means for providing at least one of a positive and negative excitation voltage to the superconducting winding based on the control signal.
Still yet another aspect of the present invention includes a method for providing DC excitation voltages to a rotating superconducting winding; including the steps of: receiving current feedback from the superconducting winding; determining an error signal based on the current feedback and a reference signal; determining a control signal corresponding to the error signal, and providing a positive or negative superconducting winding excitation voltage based on the control signal.
Another aspect of the present invention relates to a system for providing DC excitation voltages to a rotating superconducting winding; including: a system for receiving current feedback from the superconducting winding; a system for determining an error signal based on the current feedback and a reference signal; a system for determining a control signal corresponding to the error signal, and a system for providing at least one positive and negative excitation voltage to the superconducting winding based on the control signal.
To the accomplishment of the foregoing and related ends, the invention then comprises the features hereinafter fully described. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.